Behavioral experiments have shown that human rod photoreceptors have achieved the theoretical limit of sensitivity to light; absorption of a single photon of light is a detectable event. The achievement of this remarkable sensitivity begins in the rod photoreceptor itself, in the process by which light energy is transduced into an electrical signal that can be passed along to other neurons in the retina. Understanding of this visual transduction process is fundamental to an understanding of vision. The overall aim of proposed research is to gather quantitative information about the mechanism of visual transduction by measuring the electrical responses of individual photoreceptors. The electrical signal will be monitored using a new technique that is sufficiently sensitive to record the change in membrane current resulting from photoisomerization of a single molecule of rhodopsin, the visual pigment of the rods. In some of the proposed experiments, the visual transduction mechanism will be used as an amplifier to study spontaneous activation on single visual pigment molecules, and in other experiments variation in the properties of the transduction process in different regions of a single photoreceptor will be studied. Finally, the properties of cone photoreceptors in mammalian retina will be studied to determine in what ways they are similar to and in what ways different from rod photoreceptors.